The present application is directed to a process for the biological treatment of wastewater to remove organic matter, especially while reducing the phosphorus content of the effluent water without adding additional chemicals for the phosphorus removal.
Wastewater treatment has progressed substantially in the last fifty years. Early treatments utilized various chemicals to rid the wastewater of organic material. Subsequently, most chemical treatments have been surpassed by use of microorganisms which through various processes convert organic impurities in the water to various combinations of carbon dioxide, methane, water and inorganic nitrogen and phosphorus compounds.
While microorganisms are effective in reducing organic content, phosphorus and nitrogen often present problems in effluent water from biological processes. The present application is especially directed to an improved process for the removal of phosphorus from the water.
Phosphorus presents an eutrophic problem in that it substantially enhances the growth of aquatic plant life such that an influx of phosphorus may cause sufficient plant growth in streams or lakes to kill fish or produce other problems. Phosphorus is also a common component of human or animal waste and of many household or industrial products of the type that are likely to become a component of waste water collected in a city's sewer system, such as soap for washing clothes wherein phosphorus is used as whitener.
Previous processes have been developed to try to rid effluent from wastewater treatment plants of phosphorus. For example, one process wherein biomass and wastewater are mixed in an aerated tank is generally known as the Phostrip process. In the Phostrip process the biomass with some uptake of phosphorus in the bacteria is separated from the clarified water. The separated biomass is then subjected to an anaerobic condition in a thickener or stripper. In the stripper phosphorus is released by the bacteria and a phosphorus rich decant is removed from the stripper and treated with lime to remove the phosphorus. The biomass is then returned to the aerated tank to mix with incoming wastewater. This process has varying degrees of success, but requires the chemical addition of lime in order to work.
In 1974 the inventor of the present application discovered that placement of an unaerated zone or region upstream of an aerobic region in an activated sludge process would result in phosphorus uptake by the bacteria when in the aerobic region. This process is generally referred to as the Phoredox process. However, high phosphorus removal with this process was not successful with all influent waste water streams.
Subsequently, Fuhs and Chen in trying to understand the mechanism of phosphorus uptake by bacteria suggested that certain microorganisms (phosphate accumulating organisms), while obligate aerobic organisms, could take up and store certain short chain volatile fatty acids, especially acetic acid and propionic acid in an anaerobic treatment region and later use the fatty acids to take up phosphorus that is stored in the bacteria as polyphosphate in an aerobic treatment region. In theory the microorganisms store polyphosphate as an energy source in the aerobic region and release the energy stored in the polyphosphate later by breaking high energy phosphate bonds creating surplus phosphates which are released in a preceding non aerated region. Thereafter, if short chain fatty acids are available in an anaerobic region, the acids are stored in certain bacteria as an intermediate product, such as poly-β-hydroxybuterate (PHB). As the biomass passes to the aerated region, the microorganisms that have stored the organic acids metabolize the PHB and use the energy gained to again take up phosphorus from the surrounding liquid. Within the theory of this process the microorganisms will take up more phosphorus in the aerobic region than is released in the anaerobic region, if sufficient amounts of the short chain fatty acids are available. Thus, in theory, if excess biomass is wasted, then the phosphorus in the influent wastewater should be wasted with the wasted biomass. If insufficient fatty acids are present, then the phosphorus will remain outside the biomass and will be discharged with the effluent water.
While certain actual treatment facilities do receive wastewater with such short chain fatty acids in sufficient quantity to produce at least some phosphorus reduction, many have little or do not have enough to remove most or at least a substantial amount of the phosphorus. The shortage of short chain fatty acids can be made up by addition of the fatty acids from an external source, but this is a comparatively expensive and undesirable chemical addition.
Consequently, applicant has found a need for a method of biologically producing such short chain fatty acids within the biological process and has found a simple and surprisingly effective method and apparatus for doing so.